Internal combustion engines generate exhaust gases and associated noise due to the sudden expansion of combustion chamber exhaust gases. It is commonly desirable to suppress or “muffle” the noise caused by the engine. Various approaches have been used to design mufflers for limiting the sound pressure level of exhaust noise.
One conventional approach is a dissipative muffler. Dissipative mufflers generally include ducts or chambers filled with acoustic absorbing materials. These materials absorb the acoustic energy and transform it into thermal energy. However, the sound absorbing material tends to break down over time due to physical and thermal stresses, and this leads to a degradation in muffler performance. Further, when this approach creates a substantial increase in back pressure, or resistance of the muffler to the free discharge of the combustion gasses. This increase in backpressure can result in a decrease in the output horsepower of the engine with a resulting loss of efficiency in fuel economy, which are undesirable.
Another approach is a reactive muffler. Reactive mufflers generally include a number of resonating chambers of different volumes and shapes connected with pipes, and may include baffles or flow reversals. However, such configurations commonly cause increased backpressure at the exhaust of the engine that degrades engine performance.
Conventional muffler systems generally fail to attenuate sound waves over a broad band of frequencies. Mufflers typically provide effective attenuation only at specified frequencies equal to or greater than a specific cut-off frequency. Exemplary dissipative mufflers provide effective attenuation only above approximately 500 Hertz. As a result, the typical dissipative muffler fails to attenuate low frequency sound. This failure is unacceptable in an automobile exhaust muffler because the sound produced by the engine has greatest amplitude at lower frequencies, such as below approximately 500 Hertz. The transmission loss of a typical reactive muffler or expansion is characterized by a periodic series of sinusoidal “humps.” As a result, a reactive muffler provides acceptable amplitude levels of low frequency attenuation, but is by its nature tuned to a single frequency (or frequency range, or frequency characteristic), and thus provides essentially no attenuation at other frequencies.
FIG. 1 is an exploded view showing an exemplary prior art reactive muffler 10 including a tubular outer skin 20 that, when assembled, is closed at both ends by upstream end cap 23 and downstream end cap 24. Each end cap defines a respective opening 26, 28, but otherwise closes an open end of outer skin 20 when assembled. In use in an exhaust system, inlet opening 26 in upstream end cap 23 is mated to exhaust ducting to admit exhaust gases into muffler 10, and outlet opening 28 in downstream end cap 24 is mated to exhaust ducting (or directly to atmosphere), to expel exhaust gases from the muffler 10.
Sleeve tube 30 has a necked-down portion 32 at its upstream end. The outer periphery, e.g., diameter of the necked-down portion is configured to correspond to and fit closely with the inner diameter of the inlet opening 26 in the upstream end cap 23. The inner periphery, e.g., diameter, of the necked-down portion 32 is configured to correspond to and fit closely with the outer diameter of an upstream end 42 of the internal tube 40. When assembled, end cap 23, sleeve tube 30 and internal tube 40 are joined, e.g., by welding, so that gases admitted through inlet opening 26 travel through the internal tube 40.
The downstream end 44 of internal tube 40 has an outer periphery, e.g., diameter, configured to correspond to and fit closely with the inner diameter of the outlet opening 28 in downstream end cap 24. Internal tube 40 is joined to end cap 24, e.g., by welding, so that gases expelled from internal tube 40 exit the muffler 10 through outlet opening 28.
Notably, the main body portion 34 of sleeve tube 30 has an inner diameter/surface that is larger than an outer diameter/surface of internal tube 40, and the length of sleeve tube 30 is shorter than a length of internal tube 40 so that sleeve tube 30 does not abut downstream end cap 24. Further, internal tube 40 includes at least one, and preferably multiple, and more preferably a pair of opposed, openings 46, 48, defined in the sidewall of the internal tube 40.
This creates an air gap between sleeve tube 30 and internal tube 40, and a conduit that provides fluid communication from the inlet opening 26, through the internal tube 40, through openings 46 and 48, between the sleeve tube 30 and internal tube 40, and into the second volume of the skin, as well as from the second volume of the skin 20, between the sleeve tube 30 and internal tube 40, through the opening(s) 46, 48, and through the internal tube 40 to and through the outlet opening 28. This conduit provides a continuous volume, or cavity, that can be configured/tuned for noise suppression purposes. This continuous volume is defined as the internal volume of the skin 20 and end caps 23, 24.
Accordingly, this muffler provides internal structure functioning as a modified side branch resonator. By varying the relative sizes (diameters and lengths) of the sleeve and internal tubes, the air gap between them, the volume of the skin 20, and the diameter and placement of the openings in the internal tube, this muffler can be tuned as desired to attenuate a range of frequencies of exhaust-associated noise. The tuning can be performed using the Helmholtz equation to design a resonator comprised of a branch with a given length and volume of air, where the difference in diameters between the internal diameter of the sleeve tube and outer diameter of the internal tube creates a volume of air that can be modeled as a cylinder for the purpose of the Helmholtz equation, as known in the art. However, this design can be tuned to cancel out/attenuate a single frequency or range/set of frequencies according to its design. Accordingly, attenuation design involves a compromise involving design of a muffler having an external volume/configuration that fits within an available envelope of space relative to a remainder of a motor vehicle, etc., while also providing an internal volume and structures providing attenuation of a single target frequency or set of frequencies that are a function of the internal volume.
It is desirable to provide a muffler having the advantages of a reactive muffler, but providing attenuation over multiple ranges of frequencies.